net: switchdev: Add PORT_PRE_BRIDGE_FLAGS

In preparation for removing switchdev_port_attr_get(), introduce
PORT_PRE_BRIDGE_FLAGS which will be called through
switchdev_port_attr_set(), in the caller's context (possibly atomic) and
which must be checked by the switchdev driver in order to return whether
the operation is supported or not.

This is entirely analoguous to how the BRIDGE_FLAGS_SUPPORT works,
except it goes through a set() instead of get().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>